<?php
$a = rand(0,255);
$b = rand(0,255);
$c = rand(0,255);
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
       div {
           width: 300px;
       height: 300px;
       background-color: rgb(<?php echo $a; ?>,<?php echo $b; ?>,<?php echo $c; ?>);
       }
    </style>

</head>
<body>
<div>aa</div>
</body>
</html>